KL25Z: extend uart driver to handle all available ports: uart0, uart1, uart2

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
GPIO task: rewrite two functions using for loop

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
KL25Z: handle gpio irqs on port A and port D

This patch implements the handling of interrupts for GPIOs belonging to ports
A and D, which are the only one supported by KL25Z chip. For all the other
Ports (B,C,E) a polling mechanism shall be implemented, similar to the one
present for Atmega AVR.

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>